This application is based on application No. 2000-356961 filed in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a diffractive optical element, and more particularly, to a diffractive optical element in which a dielectric multilayer film is provided on a diffraction grating.
2. Description of the Prior Art
Diffractive optical elements that diffract light are used in various types of apparatus that handle light, such as optical pickups, optical communication devices, laser beam printers, copiers and microscopes. Diffractive optical elements include a transmissive type that transmits light and diffracts the transmitted light, and a reflective type that reflects light and diffracts the reflected light. Both types comprise a substrate on a surface of which a diffraction grating, comprising periodically repeated minute depressions and projections, is formed. In reflective type diffractive optical elements, a reflecting film of a metal such as aluminum is provided on the diffraction grating. In transmissive type diffractive optical elements and reflective type optical elements used with light incident from the reverse side of the substrate, a transparent substrate is used.
Diffraction gratings are broadly divided into a blaze type with an inclined surface and a binary type with a horizontal surface. Blaze type diffraction gratings include one with a unidirectional surface inclination and one with a bidirectional surface inclination. Although both are triangular in cross section, the former has a sawtooth cross section and the latter has a V-shaped cross section. Binary type diffraction gratings include a two-level grating with only the highest level and the lowest level, and a multilevel grating with levels intermediate the highest and the lowest levels. The former has a rectangular cross section, and the latter has a step-shaped cross section comprising a set of rectangles. In blaze type diffraction gratings with a unidirectional surface inclination and two-level binary type diffraction gratings, a level difference perpendicular to the entire surface of the substrate is present on the borders between the depressions and projections. In multilevel binary type diffraction gratings, a perpendicular level difference is also present on the borders between the levels.
In recent years, it has been proposed to provide on the diffraction grating a dielectric multilayer film formed by alternately and periodically laminating dielectric layers with a high reflective index and dielectric layers with a low refractive index to thereby improve the reflectance of the diffractive optical element and improve the wavelength selectivity and the polarization selectivity of reflection. Such a diffractive optical element, which can function as the transmissive type or function as the reflective type, is high in utility.
An example of a diffractive optical element in which a dielectric multilayer film is provided on the diffraction grating is shown in FIG. 4. This diffractive optical element 5 comprises a substrate 51 and a dielectric multilayer film 53 comprising two kinds of dielectric layers 53a and 53b that are alternately laminated. A diffraction grating 52 is formed on a surface of the substrate 51. The diffraction grating 52 is a blaze type with a unidirectional surface inclination.
In a case where reflected light is diffracted, the first-order diffraction efficiency is highest when the optical level difference between the depressions and projections of the diffraction grating is xc2xd the wavelength of the light. Therefore, in a case where light is made incident from the reverse side of the substrate to obtain first-order reflected light, the physical level difference h0 between the depressions and projections of the diffraction grating is set to h0=xcex/2n0 where the wavelength of the light to be reflected is xcex and the refractive index of the substrate is n0. In a case where light is made directly incident on the dielectric multilayer film from the obverse side of the substrate, that is, from an air interface, to obtain first-order reflected light, since the refractive index of air is 1, the physical level difference h0 is set to h0=xcex/2.
The reflectance of the dielectric multilayer film is highest when the optical thicknesses of the layers are xc2xc the wavelength of the light. Therefore, the physical thicknesses h1 and h2 of the two kinds of layers of the dielectric multilayer film are set to h1=xcex/4n1 and h2=xcex/4n2, where the refractive indices of the layers are n1 and n2.
By setting the diffraction grating and the dielectric multilayer film as described above in consideration of the wavelength of the light to be reflected, the wavelength selectivity is improved, and the desired light can be efficiently extracted as the reflected light and can be prevented from being mixed in transmitted light of a different wavelength. In addition, the polarization selectivity of a dielectric multilayer film that transmits p-polarized light and reflects s-polarized light is improved, so that separation between p-polarized light and s-polarized light is ensured.
However, in the conventional diffractive optical element in which the dielectric multilayer film is provided on the diffraction grating, although the physical level difference h0 between the depressions and projections of the diffraction grating and the physical thicknesses h1 and h2 of the two kinds of layers of the dielectric multilayer film are set as mentioned above, these are individually decided and no consideration is given to the relationship among the level difference h0 between the depressions and projections of the diffraction grating and the thicknesses h1 and h2 of the layers of the dielectric multilayer film. For this reason, in a structure in which the diffraction grating has a level difference, layers of the dielectric multilayer film having different refractive indices are in contact with each other at the level difference of the grating, so that the reflectance decreases.
This problem will be described with the diffractive optical element 5 of FIG. 4 as an example. Assuming now that the refractive index n0 of the substrate 51 is 1.5 and the refractive indices n1 and n2 of the layers 53a and 53b of the dielectric multilayer film 53 are 2.5 and 1.875, respectively, a relationship xcex/2n0=xcex/4n1+xcex/4n2+xcex/4n1 exists. That is, h0=2h1+h2, and the physical level difference h0 between the depressions and projections of the diffraction grating 52 is the sum of twice the physical thickness h1 of the layers 53a of the dielectric multilayer film 53 and the physical thickness h2 of the layers 53b. Therefore, odd-numbered layers 53a with a high refractive index and even-numbered layers 53b with a low refractive index are in contact with each other in the region of the level difference G of the diffraction grating 52.
When light is obliquely incident on the diffractive optical element 5 from the reverse side of the substrate 51 (as depicted by the arrow) and reaches the area of the level difference G of the diffraction grating 52, the effective thicknesses h1 and h2 of the layers 53a and 53b for the light are shifted from xcex/4n1 and xcex/4n2 where the reflectance is highest, and become smaller than xcex/4n1 and xcex/4n2 or become twice xcex/4n1 and xcex/4n2, for example, xcex/2n1 and xcex/2n2 for the case of the light shown by the arrow. Consequently, the reflectance of the dielectric multilayer film 53 decreases, so that the light passes through the diffractive optical element 5 rather than being reflected.
Since light perpendicularly incident from the reverse side of the substrate 51 does not obliquely traverse the film in the area of the level difference G, it appears that the reflectance of the dielectric multilayer film 53 does not decrease for such light. However, the diffracted reflected light obliquely traverses the level difference G because it is not parallel to the level difference G, so that the reflectance decreases.
The decrease in the reflectance of the dielectric multilayer film 53, resulting from the layers 53a and 53b with different diffractive indices being in contact with each other at the level difference G of the diffraction grating 52, occurs not only when light is incident from the reverse side of the substrate 51, but also when light is directly incident on the dielectric multilayer film 53 from the obverse side of the substrate 51.
When the reflectance decreases, problems occur such that the quantity of the reflected light to be extracted decreases and that unnecessary light is mixed in the transmitted light to be extracted. Although the decrease in the reflectance of the dielectric multilayer film can be avoided by increasing the number of laminations of layers with a high refractive index and layers with a low refractive index, doing this increases the number of processes and the time for film formation, so that the manufacturing efficiency of the diffractive optical element significantly decreases.
An object of the present invention is to provide an improved diffractive optical element.
Another object of the present invention is to improve the reflectance of a diffractive optical element having a dielectric multilayer film on a diffraction grating having a level difference.
The above-mentioned objects are attained by a diffractive optical element comprising: a substrate on a surface of which a diffraction grating is formed; and a dielectric multilayer film provided on the diffraction grating on the surface of the substrate and in which the diffraction grating has a level difference substantially perpendicular to the entire surface of the substrate and the dielectric multilayer film is continuous across the level difference of the diffraction grating, such that only the same kind of dielectric layers included in the dielectric multilayer film are continuous across the level difference of the diffraction grating.
In this diffractive optical element, although the diffraction grating has a level difference, since different kinds of dielectric layers included in the dielectric multilayer film are not in contact with each other at the level difference of the diffraction grating, even when there is light obliquely traversing the area of the level difference, the effective thicknesses of the layers for the light do not change. Consequently, high reflectance is obtained.
Moreover, the above-mentioned objects are attained by a diffractive optical element comprising: a substrate on a surface of which a diffraction grating is formed; and a dielectric multilayer film provided on the diffraction grating on the surface of the substrate and in which the diffraction grating has a level difference substantially perpendicular to the entire surface of the substrate and the dielectric multilayer film is continuous across the level difference of the diffraction grating, wherein the size of the level difference of the diffraction grating is an integral multiple of the thickness of one period of the dielectric multilayer film.
When the size of the level difference of the diffraction grating is an integral multiple of the thickness of one period of the dielectric multilayer film, only the same kind of dielectric layers are continuous across the level difference of the diffraction grating. Consequently, high reflectance is obtained. Setting the size of the level difference of the diffraction grating to be an integral multiple of the thickness of one period of the dielectric multilayer film can be easily realized depending on the setting of the refractive indices of the layers of the dielectric multilayer film.
Moreover, to solve the above-mentioned objects, according to the present invention, in a diffractive optical element comprising a substrate on a surface of which a diffraction grating is formed; and a dielectric multilayer film provided on the diffraction grating on the surface of the substrate and in which the diffraction grating has a level difference substantially perpendicular to the entire surface of the substrate and the dielectric multilayer film is continuous across the level difference of the diffraction grating, the size of the level difference of the diffraction grating is an integral multiple of the thickness of one period of the dielectric multilayer film.
When the size of the level difference of the diffraction grating is an integral multiple of the thickness of one period of the layers included in the dielectric multilayer film, only the same kind of dielectric layers are continuous across the level difference of the diffraction grating. Consequently, high reflectance is obtained. Setting the size of the level difference of the diffraction grating to an integral multiple of the thickness of one period of the dielectric multilayer film can be easily realized depending on the setting of the refractive indices of the layers of the dielectric multilayer film.
In this case, the dielectric multilayer film has only two kinds of layers in one period, and the relationship of the following equation (1) is satisfied:
2/n0=m(1/n1+1/n2)xe2x80x83xe2x80x83(1)
where n0 is the refractive index of the substrate, n1 and n2 are the refractive indices of the two kinds of layers of the dielectric multilayer film, and m is an integer not less than 1.
The dielectric multilayer film having only two kinds of layers in one period has the simplest structure, and is easy to manufacture. By satisfying equation (1), the size of the level difference of the diffraction grating is an integral multiple of the thickness of one period of the dielectric multilayer film, and at the same time, the relationships h0=xcex/2n0, h1=xcex/4n1, and h2=xcex/4n2 hold. Therefore, when light is incident from the reverse side of the substrate, first-order reflected light can be efficiently obtained.
The dielectric multilayer film may have only two kinds of layers in one period and satisfy the relationship of the following equation (2):
2=m(1/n1+1/n2)xe2x80x83xe2x80x83(2)
where n1 and n2 are the refractive indices of the two kinds of layers of the dielectric multilayer film, and m is an integer not less than 1.
By satisfying equation (2), the size of the level difference of the diffraction grating is an integral multiple of the thickness of one period of the dielectric multilayer film, and at the same time, the relationships h0=xcex/2, h1=xcex/4n1 and h2=xcex/4n2 hold. Therefore, when light is directly incident on the dielectric multilayer film from the obverse side of the substrate, first-order reflected light can be efficiently obtained.